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Renesas Technology America Inc.
An Introduction to CANAn Introduction to CAN
CAN Basics 2CAN Basics 2Renesas InteractiveRenesas Interactive
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Key reasons to use CAN
• Scalability• Low wiring cost• Low node connection costs
• More and more chips• More off the shelf tools• Knowledge base growing• Reliability (error free)
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CAN Message Frames
• Data Frame - Carries the actual data
• Error Frame - When error is detected by any node’shardware
• Overload Frame - Request a delay on the bus
• Interframe Space - Provides minimum spacing between data and remote frames Allows error frames priority
• Remote Frame - Request transmission of Data Frame.
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Data Frame is broadcast to the bus [id_n][value_x]
Transmitting Node
MCU Firmware
identifier [id_n]
Data [values_x]
CAN Peripheral
CAN Transceiver
Node configured to receive identifier
MCU Firmware
CAN Transceiver
CAN Peripheral
Rx Mail Box [id_n]
Rx Mail Box [id_b]
Tx Mail Box [id_c]
Data [values_x]
Data [values_x]
Tx Mail Box [id_n]
Rx Mail Box [id_c]
Rx Mail Box [id_b]
Data [values_x]
Node not configured to receive identifier
MCU Firmware
CAN Transceiver
CAN Peripheral
Rx Mail Box [id_d]
Tx Mail Box [id_b]
Rx Mail Box [id_c]
Rx Mail Box [id_a]
Data Flow
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Data Frame
• Start of Frame
• Arbitration Field
• Control Field
• Data Field
• CRC
• ACK Field
• End of Frame
re
se
rve
d 0
re
se
rve
d 0
AC
K D
elim
ite
r
AC
K D
elim
ite
r
Sta
rt
of
Fra
me
Re
mo
te R
eq
ue
st
ID E
xte
nd
Da
ta L
en
gth
C
od
e (
4 b
its)
Data Field0 to 8 bytes
CR
C D
elim
ite
rA
CK
slo
t
End of Frame
Sta
rt
of
Fra
me
Su
bstitu
te R
RID
Exte
nd
Da
ta L
en
gth
C
od
e (
4 b
its)
Data Field0 to 8 bytes
CR
C D
elim
ite
rA
CK
slo
t
End of Frame
Re
mo
te R
eq
ue
st
Standard ID
Extended ID
CRC
CRC
Identifier
Identifier(second part)
re
se
rve
d 1
Identifier(first part)
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Start of Frame
• A single dominant bit while the bus is idle• All nodes synchronize timing to leading edge
All nodes synchronize to
edge
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Arbitration Field
Standard; 11-bit identifier
Extended; 29-bits
8
Arbitration
Carrier Sense Multiple Access/Collision Resolution= CSMA/CR
By using Dominant and Recessive Bits
Handling collisions
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Control Field
Control Field contains the DLC• 2 bits reserved• 4 bits Data Length Code; number of Data Field bytes
Number of Data Bytes
Data Length Code
DLC3 DLC2 DLC1 DLC0
0 0 0 0 0
1 0 0 0 1
2 0 0 1 0
3 0 0 1 1
4 0 1 0 0
5 0 1 0 1
6 0 1 1 0
7 0 1 1 1
8 1 0 0 0
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Data Field
• Data content = payload
of the frame• 0-8 bytes• MSB transmitted first
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CRC Field
• 15 bit CRC value followed by a recessive delimiter• Generated by transmitter node’s HW and verified in
receiver’s HW• CRC polynomial: x15+x14+x10+x8+x7+x4+x3+1
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ACK Field
ACK FIELD = ACK SLOT + ACK DELIMITER
All receivers that received valid message reports with a ’dominant’ bit during the ACK slot
Any receiver that disagrees votes no after the delimiter with an error flag
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End of Frame
DATA FRAME and REMOTE FRAME delimited by ‘End of Frame’ consisting of seven recessive’ bits.
Gives a break before the next frame..
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Error detection
Locally detected errorsWhen is an error frame sent?
• Bit error -receive bit at sender not equal to transmit bit.
• Bit stuffing violation -max 5 bits with same polarity
• CRC error -checksum violation
• Form error -bit pulse malshaped
• Ack error -no dominant level in ACK slot so sender must retransmit
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What is needed?
• Firmware• CAN controller • CAN transceivers• A physical media
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Example of Renesas CAN MCUs• SH705x series
• Automotive Powertrain• SH7047 series
• General purpose SH2…soon updated derivative available.• M32R/ECU series
• Automotive Powertrain• M32C series
• General purpose (1-3 channels of CAN)• M16C/6N series
• M16C/62 family with CAN. General purpose.• M16C/29 series
• General Purpose M16C/Tiny with CAN.• M16C/10 series
• Better off with M16C/Tiny or R8C/Tiny• R8C/Tiny series
• General Purpose
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Which is normally by far the most common frame type?
1. Data Frame
2. Error Frame
3. Overload Frame
4. Remote Frame
A: 1
B: 2
C: 3
D: 4
Question 1
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Which of these statements is false?
1. The data field in a data frame can consist of max 255 bytes
2. The data field has only 8 bytes
3. You must specify for each data frame how many data bytes follow
4. Remote frames are not necessary to use CAN
A: 1
B: 2
C: 3
D: 4
Question 2
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Which of these is false?
1. The CRC field is sent by all receivers and verified by the transmitter.
2. Each receiver generates a CRC on the observed data frame and compares it with the transmitted CRC check value.
3. Every receiver which has received a valid message correctly, reports this to the transmitting node with a ’dominant’ bit during the ACK slot. Any node that disagrees, votes no after the delimiter by sending an error flag.
A: 1
B: 2
C: 3
Question 3
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When is an error frame sent?
1. Bit stuffing violation; > 5 bits with same polarity
2. CRC error; checksum violation
3. Form error; bit pulse malshaped
4. Ack error; no dominant level in ACK slot so sender must retransmit
A: 1-3
B: 2-4
C: 2 and 4
D: All above
Question 4
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Glossary• Advanced CAN: CAN peripheral with varying numbers of buffers configurable for transmit/receive. Receive buffers have
hardware filtering on at least mask/match identifier content.• Basic CAN: CAN peripheral with no hardware filtering. Typically two receive buffers act as FIFO accepting all bus traffic. Usually
one transmit buffer.• Bit Time: Nominal time of one bit on the CAN bus. Made up of multiple segments that allows each node to synchronize to the
received bus traffic. All nodes on a bus must be configured to the same (nominal) bit time.• CAN: Controller Area Network• CAN 2.0B: Version 2.0 was the last version of CAN defined by Bosch. Part B added extended identifiers and the idea of
hardware filtering.• CIA: CAN in Automation. Group controlling the CANOpen protocol.• CANOpen: Multi-area communication protocol using CAN.• CRC: Cyclic Redundancy Check• DeviceNet: Industrial communication protocol using CAN.• Dominant/Recessive: Dominant bits on physical layer can override recessive bits.• Filters: Hardware in the CAN peripheral that can mask/match bits within the identifier field used to determine whether or not route
bus data to a mailbox.• GM LAN 3.0: GM protocol, encompasses all GM serial protocols.• Identifier: Frame field that indicates the message content. This field also is used to arbitrate the message priority on the bus;
lower identifier is higher priority.• Standard Format; frames use 11 bit identifier.• Extended Format “- 29 bits
• ISO 11898: ISO standardized version of CAN.• Mailbox: CAN hardware buffer that can be used to transmit or receive data. Most full CAN implementations have at least 16
mailboxes.• ODVA: Open DeviceNet Vendor Association. Group controlling DeviceNet protocol.• Time Quanta: Smallest time unit used by CAN. Multiple time quanta make up the segments of a bit time.• TT CAN: Time Triggered CAN. More deterministic CAN by assigning time slots when nodes may transmit.• FlexRay: Next generation automotive network. Time slots on the bus provide more deterministic behavior.• Vector-CanTech: Supplier of the majority of CAN software drivers and tools to North American and European automobile ECUs.
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Come back for more!
End CAN Basics 2End CAN Basics 2
